Method and apparatus for liquefaction of co2

ABSTRACT

In a method for the liquefaction of a gas containing at least 60 mol % of CO2, in order to produce at least one liquid product, a first feed gas is compressed from a first pressure to a second pressure and cooled so as to form a liquid or supercritical flow, at least part of the liquid or supercritical flow is cooled in a heat exchanger in order to form a cyclic liquid at the second pressure, the cyclic liquid is divided into at least two fractions, including an auxiliary fraction, at least one fraction being vaporized in the exchanger by means of heat exchange with the liquid or supercritical flow part, and where there are at least two fractions, each one is vaporized at a different pressure, and the at least one gas formed is then compressed and mixed with the first feed gas, the auxiliary fraction either forming the liquid product or one of the liquid products, or being treated by separation at a sub-ambient temperature in at least one separating means in order to form the liquid product.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT Application PCT/FR2012/050562, filed Mar. 16, 2012, which claims the benefit of FR 1152164, filed Mar. 16, 2011, both of which are herein incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and apparatus for liquefaction of CO₂.

BACKGROUND

An industrial method envisaged for the transport of CO₂ is that of transport by boat, requiring liquefaction of the CO₂, for example issuing from various sources: gas from coal-fired powered stations, steelmaking units, SMRs, gasification processes, etc.

It is sometimes necessary to transport the CO₂ by pipes at supercritical pressure and for this purpose the liquid to be transported must be pressurised at high pressures.

This liquefaction may be preceded by one or more fume (or synthesis gas) treatments by physical and/or chemical separation methods.

U.S. Pat. No. 4,699,642 describes a method for the liquefaction of a gas rich in CO₂ according the preamble of claim 1.

In this method, the cycle liquid vaporises at a pressure substantially equal to that of the gas to be liquefied.

Liquefying a flow of CO₂ by compressing it and cooling it is known according to JP-A-58208117, EP-A-0646756 and SU-A-1479802.

SUMMARY OF THE INVENTION

The invention proposes optimised schemes for the liquefaction of CO₂ and optionally the conditioning of the CO₂ for transport by pipes from one of more sources, minimising the losses of CO₂ as well as the specific energy related to the purification and liquefaction of the CO₂.

According to the invention, the impure CO₂ issuing from one or more sources is compressed via the cycle compressor to the desired pressure for condensing the CO₂ at ambient temperature (or against an intermediate cold fluid). Part of this CO₂ may be directly compressed at a sufficient pressure for the transport of the CO₂ by pipes, another part of the CO₂ may be used in the cold box. The liquid CO₂ sent into the cold box has potentially two uses: one part may be purified for the production of liquid CO₂, the other provides the refrigeration balance by expansion of the liquid CO₂. The unit may also comprise the following technological stages:

-   -   Drying of the gas by adsorption upstream of the compressor.     -   Elimination or reduction of impurities such as Hg by adsorption,         NOx via a distillation column.     -   Purification of the CO₂ via a distillation column.     -   Improvement of the CO₂ yield via an intermediate compression in         the cold box.

This arrangement thus allows reliable use of the liquefier even if one of the CO₂ sources is lost.

If at least one source of impure CO₂ is at the correct pressure for liquefaction of the CO₂, the gas of this source is preferentially chosen for the production of liquid CO₂.

Optionally the compression of the gas from one of the sources may be necessary for equalling at least the pressure of the liquid CO₂ produced.

According to one subject matter of the invention, a method is provided for the liquefaction of a gas containing at least 60% mol. of CO₂, or even at least 80% CO₂, for producing at least one liquid product in which a feed gas is compressed from a first pressure to a second pressure and cooled in order to form a liquid or supercritical flow, at least part of the liquid or supercritical flow is cooled in a heat exchanger in order to form a cycle liquid at the second pressure, the cycle liquid or a liquid derived from the cycle liquid is divided into at least two fractions including an auxiliary fraction, at least one fraction being vaporised in the exchanger by exchange of heat with the part of the liquid or supercritical flow, the at least one fraction being vaporised, and where there are at least two fractions each is vaporised at a different pressure, and the gas or gases formed then being compressed and mixed with the first feed gas, the auxiliary fraction either constituting the liquid product or one of the liquid products or being treated by separation at sub-ambient temperature in at least one separation means in order to form the liquid product or one of the liquid products, characterised in that the feed gas is compressed from the first pressure to the second pressure after having been mixed with at least one gas formed by vaporisation of the cycle liquid, the gas formed by vaporisation having been compressed from its vaporisation pressure to the first pressure without having been mixed with the first gas.

Thus the cycle liquid vaporises at a lower pressure than the first pressure.

Preferably the cycle liquid vaporises at two different pressures, each lower than the first pressure.

Optionally:

the liquid derived from the cycle liquid is derived by separation in a phase separator to produce a gaseous fraction and a liquid fraction, the liquid fraction constituting the liquid derived from the cycle liquid;

the cycle liquid vaporises in the exchanger at the same composition as the liquid or supercritical flow cooled in the exchanger;

the fraction of the liquid or supercritical flow is not cooled in the heat exchanger and serves as a product;

a second feed gas is purified in the separation means or at least one of the separation means, without having been compressed with the first feed gas;

at least one fraction of the cycle gas is vaporised at a pressure lower than the first pressure and compressed to the first pressure before being mixed with the first feed gas;

the auxiliary fraction and/or the second feed gas is/are purified in order to form the liquid product or one of the liquid products by separation in at least one phase separator and/or at least one distillation column;

the feed gas mixed with the cycle gas at the second pressure is cooled upstream of the heat exchanger in order to supply the liquid or supercritical flow and the liquid or supercritical flow is next sent to the heat exchanger;

the feed gas at the second pressure is cooled upstream of the heat exchanger in order to form a single liquid or supercritical flow and no gaseous flow;

the liquid or supercritical flow is sub-cooled in the exchanger (E1).

According to another subject matter of the invention, apparatus is provided for the liquefaction of a gas containing at least 60% mol. CO₂ in order to produce at least one liquid product, comprising a heat exchanger, a compression means comprising at least two stages in series, with a first stage and a second stage downstream of the first stage, a pipe for sending a feed gas to the inlet of the second stage of the compression means in order to be compressed from a first pressure to a second pressure, means for cooling the gas at the second pressure in order to form a liquid or supercritical flow, optionally a pipe for bringing some of the liquid or supercritical flow to serve as a product without passing through the heat exchanger, a pipe for bringing at least part of the liquid or supercritical flow to cool in the exchanger in order to form a cycle liquid at the second pressure, at least one pipe for sending at least a fraction of the liquid to vaporise in the exchanger, for the case where several fractions vaporise therein means for vaporising the at least two fractions each at a different pressure, and at least one pipe for sending the gas or gases formed to the compression means and a pipe for an auxiliary fraction of cycle liquid connected to the heat exchanger in order to transport either the or one of the liquid products or a liquid to be treated in order to form the or one of the liquid products, characterised in that the pipe for sending the gas formed to the compression means is a pipe for sending at least one gas formed at the inlet of the first stage in order to form a compressed gas, the apparatus comprising a pipe for sending the gas compressed in the first stage to the second stage.

According to other optional features:

at least one said pipe for sending a gas formed by vaporisation to the compression means is connected to a point of the compression means upstream of the entrance of the first feed gas into the compression means;

the apparatus comprises a cold box containing the exchanger and means for depressurising a liquid to be vaporised in the exchanger, as well as means for treating the auxiliary fraction of the cycle liquid, consisting of at least one phase separator and/or at least one distillation column;

the apparatus comprises a pipe for sending a second feed gas to the treatment means without having been compressed in the compression means;

the pipe for sending a second feed gas to the treatment means is connected to the exchanger;

the exchanger consists of a single exchanger body;

the apparatus does not comprise any means for enriching the cycle liquid with CO₂ before dividing it.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 shows an embodiment of the present invention.

FIG. 2 shows an embodiment of the present invention.

FIG. 3 shows an embodiment of the present invention

DETAILED DESCRIPTION

The invention will be described in more detail with reference to the figures.

In FIG. 1, the apparatus comprises a heat exchanger E1 made from brazed aluminium of the plate type, a pump 42, a compressor with four stages C1, C2, C3, C4 and a series of phase separators P1, P2, P3.

The phase separators, the heat exchanger and the expansion valves are situated in a cold box.

A mixture of gas from three different sources is liquefied to form a flow of supercritical and purified CO₂ in order to form a liquid flow of pure CO₂.

The gas 1 containing at least 60% mol CO₂, or even at least 90% mol CO₂ or optionally at least 95% mol CO₂, as well as at least one other gas, which may for example be nitrogen, methane, or carbon monoxide, is to be liquefied. In this quantified example, the gas contains 99% CO₂ and 1% nitrogen. The gas 1 at a first pressure is mixed with the gases 1A, 1B and the mixture formed is sent to the third stage C3 of a four-stage compressor. The gas is cooled by the cooler R3, compressed in the fourth stage C4 to a second pressure beyond the critical pressure, for example 83 bar absolute, and then cooled in the exchangers E3, E4 in order to form a supercritical fluid. Optionally a part 40 of the fluid is not sent to the heat exchanger E1 but is pressurised by the pump 42 to a pressure of 150 bar in order to form a product, for example to be sent in a pipe. The rest of the fluid or the fluid at the output pressure of the stage C4 is cooled to approximately −50° C. in the exchanger E1 in order to form a cycle liquid, since it is no longer supercritical. The liquid is divided into optionally five fractions. A fraction at very high pressure 4 is expanded in the valve 6, vaporised in the exchanger E1 and sent to the third stage C4 of the compressor. A fraction at high pressure 5 is expanded in the valve 15, vaporised in the exchanger E1 and sent to the third stage C3 of the compressor. A fraction at medium pressure 7 is expanded in a valve, vaporised in the exchanger E1 and sent to the inlet of the second stage C2 of the compressor. A fraction at low pressure is expanded in a valve 43 at 6.9 bar absolute and sent to a phase separator 35. This low-pressure fraction will be at 5.6 bar absolute if the nitrogen concentration is reduced to approximately 100 ppm; the factor limiting the reduction in pressure is the temperature: it is necessary to avoid this fluid being too cold (for example −54.5° C.) in order to prevent its approaching the temperature at which CO₂ will freeze. The only thing that is necessary is the vaporisation of a flow in the exchanger, for example the vaporisation of the low-pressure fraction. This being the case, vaporisation at several pressures improves the exchange and reduces the energy consumption. The liquid formed 39 and the gas formed 37 are vaporised and heated in the exchanger E1, mixed and sent to the inlet of the first stage C1 of the compressor.

The rest of the liquid 13 is expanded in a valve 19 (or a liquid turbine), without passing through the exchanger E1, and sent to a phase separator P1. In the phase separator, it forms a liquid 23 and a gas 21. The liquid 23 is heated in the exchanger and then sent to the third phase separator P3. The liquid from this separator is the product rich in carbon dioxide 25 of the apparatus, at −50° C. and 7 bar absolute. The gases from the separators P1 and P3 are mixed, cooled in the exchanger E1, compressed by a compressor C5, for example at 20 bar absolute, cooled in a cooler 31 and then partially condensed in the exchanger E1 and sent to the second phase separator P2. The gas 33 from this separator contains 39% carbon dioxide and 61% nitrogen and is heated in the exchanger E1. The liquid is heated in the exchanger as a flow 36 and then mixed with the fraction 13 sent to the separator P1.

It is obvious that the method does not necessarily comprise the treatment of the fraction 13 or of the gaseous phases from the pots P1 and P3 and therefore that these gaseous fractions may be simply sent to the compressor after heating in the exchanger E1. In this case, the liquid 20 and/or the liquids 22 or 25 constitute one of the liquid products of the method.

It is also possible to treat the fraction 13 by distillation in order to produce a liquid product rich in carbon dioxide or to treat it quite simply in order to remove a gaseous part.

According to FIG. 2, the elements already described in FIG. 1 will not be described again. Unlike the method of FIG. 1, a portion 1 of the feed flow is already at a sufficiently high pressure to be sub-cooled in the exchanger E1. Two flows 1A, 1B at the first pressure are therefore compressed to the second pressure by being sent to the third stage of the compressor, as described previously, and the other flow 1 at the second pressure is sent directly to the exchanger E1, where it is sub-cooled at least partially before being sent to the phase separator. It will be appreciated that the flow 1 may be the only flow treated in the phase separator or otherwise that it may be mixed with the fraction 13 after expansion. This way of proceeding also makes it possible to manage the case in which the flow 1 and the flows 1A, 1B have very different purities. For example, the flow 1 may be sent either to the separator P1 or to the separator P2 or to the separator P3 according to its composition.

According to FIG. 3, otherwise than in FIG. 2, even if the flow 1 must be compressed in an auxiliary compressor C6 in order to attain the necessary pressure, it is is possible to send gas from a source directly to the treatment by purification and other gases to the CO₂ liquefaction cycle. This way of proceeding also makes it possible to manage the case in which the flow 1 and the flows 1A, 1B have very different purities. For example, the flow 1 may be sent either to the separated P1 or to the separator P2 or to the separator P3 according to its composition.

It will be noted that the liquid flow cooled in the exchanger E1 is divided into four parts 4, 5, 7 and 41, which are then heated in the exchanger E1 in order to be sent to the compressor C1, C2, CS, C4, in addition to the flow intended for the liquid production. There are therefore as many vaporised flows each at a different pressure as there are compressor stages, each vaporisation pressure corresponding to a compressor stage input pressure. This corresponds to an optimum and it is obviously possible to vaporise only one flow intended for the compressor (for example the one at the lowest pressure) or fewer flows than there are compressor stages.

According to the desired purities, it is possible to recover liquid 20 coming directly from the exchanger E1 and not to treat the rest of the liquid in the phase separator or to treat it, according to the requirements of the customer.

It is also possible to recover liquid 22 at the output from the phase separator P1 as the only product or as one of the products.

Finally, the liquid 25 coming from the phase separator P3 may be the only one or one of the products.

In order to obtain purer products, at least one of the phase separators P1, P3 may be replaced by a distillation column.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

1-15. (canceled)
 16. A method for liquefaction of a gas containing at least 60% mol. of CO₂ to produce at least one liquid product, the method comprising the steps of: cooling and compressing a feed gas from a first pressure to a second pressure to form a liquid or supercritical flow; cooling at least part of the liquid or supercritical flow in a heat exchanger to form a cycle liquid at the second pressure; dividing the cycle liquid or a liquid derived from the cycle liquid into at least two fractions including an auxiliary fraction, wherein at least one fraction is vaporized in the exchanger by exchange of heat with the part of the liquid or supercritical flow, and vaporizing at least two fractions each at a different pressure and then compressing and mixing the formed gas or gases with the first feed gas, wherein the auxiliary fraction either constituting the liquid product or one of the liquid products or being treated by separation at sub-ambient temperature in at least one separation means in order to form the liquid product or one of the liquid products, wherein the feed gas is compressed from the first pressure to the second pressure after having been mixed with at least one gas formed by vaporization of the cycle liquid, the gas formed by vaporization having been compressed from its vaporization pressure to the first pressure without having been mixed with the first gas.
 17. The method as claimed in claim 16, in which the liquid derived from the cycle liquid is derived by separation in a phase separator in order to produce a gaseous fraction and a liquid fraction, the liquid fraction constituting the liquid derived from the cycle liquid.
 18. The method as claimed in claim 16, in which a fraction of the liquid or supercritical flow is not cooled in the heat exchanger and serves as a product.
 19. The method as claimed in claim 16, in which a second feed gas is purified in the separation means or at least one of the separation means, without having been compressed with the first feed gas.
 20. The method as claimed in claim 16, in which at least one fraction of the cycle gas is vaporized at a pressure lower than the first pressure and compressed to the first pressure before being mixed with the first feed gas.
 21. The method as claimed in claim 16, in which the auxiliary fraction and/or the second feed gas is/are purified in order to form the liquid product or one of the liquid products by separation in at least one phase separator and/or at least one distillation column.
 22. The method as claimed in claim 16, in which the feed gas mixed with the cycle gas at the second pressure is cooled upstream of the heat exchanger in order to supply the liquid or supercritical flow and the liquid or supercritical flow is next sent to the heat exchanger.
 23. The method as claimed in claim 22, in which the feed gas at the second pressure is cooled upstream of the heat exchanger in order to form a single liquid or supercritical flow and no gaseous flow.
 24. The method as claimed in claim 22, in which the liquid or supercritical flow is sub-cooled. in the exchanger.
 25. An apparatus for liquefaction of a gas containing at least 60% mol. CO2 in order to produce at least one liquid product, the apparatus comprising: a heat exchanger; a compression means comprising at least two stages in series, a first stage and a second stage downstream of the first stage; a pipe configured to send a feed gas to the inlet of the second stage of the compression means in order to be compressed from a first pressure to a second pressure; means for cooling the gas at the second pressure in order to form a liquid or supercritical flow; a pipe configured to bring at least part of the liquid or supercritical flow to cool in the exchanger in order to form a cycle liquid at the second pressure; at least one pipe configured to send at least a fraction of the liquid to vaporize in the exchanger; at least one pipe configured to send the gas or gases formed to the compression means; a pipe for an auxiliary fraction of cycle liquid connected to the heat exchanger in order to transport either one of the liquid products or a liquid to be treated in order to form one of the liquid products; and a pipe configured to send the gas compressed in the first stage to the second stage, wherein the pipe configured to send the gas formed to the compression means is a pipe configured to send at least one gas formed at the inlet of the first stage in order to form a compressed gas.
 26. The apparatus as claimed in claim 25, in which at least one said pipe configured to send a gas form by vaporization to the compression means is connected to a point of the compression means upstream of the entrance of the first feed gas in the compression means.
 27. The apparatus as claimed in claim 25, comprising a cold box containing the exchanger and means of expanding a liquid to be vaporized in the exchanger, as well as means for treating the auxiliary fraction of the cycle liquid, consisting of at least one phase separator and/or at least one distillation column.
 28. The apparatus as claimed in claim 27, comprising a pipe configured to send a second feed gas to the treatment means without having been compressed in the compression means.
 29. The apparatus as claimed in claim 28, in which the pipe configured to send a second feed gas to the treatment means is connected to the exchanger.
 30. The apparatus as claimed in claim 25, further comprising a pipe configured to bring some of the liquid or supercritical flow to serve as a product without passing through the heat exchanger.
 31. The apparatus as claimed in claim 25, further comprising means for vaporizing the at least two fractions each at a different pressure for the case where several fractions vaporize therein. 